Air circulation for ventilation of mine operations is achieved by means of a pressure drop in the mine produced by a ventilation fan either blowing air into the mine, or pulling air out of the mine. Many underground headings, i.e. main corridors, are arranged to carry air either from the surface to the workplace, or other areas requiring ventilation, or carry exhaust air back to the surface. Room and pillar mines, as commonly employed in coal mining, must, for safety reasons, have "cross cuts" or connecting paths between all headings. By law in the United States, cross cuts are required at least every 100 feet. This includes main intake and exhaust headings. In order to maintain the desired circulation path of ventilation air, these cross cuts are sealed by a partition or wall called a "stopping", typically constructed of concrete block, to prevent short circuit of the air flow. Many of these stoppings (by law in the United States, every other one) must contain a door to allow passage of personnel between the intake and the exhaust headings.
Each door, typically 32 inches square, must be manually operable by a miner and yet automatically remain closed to provide a seal against short circuiting of the air flow. Due to the pressure drop across the access opening caused by the flow of ventilation air, the force required to open these doors may be relatively large, e.g., assuming a typical door hinged from above, a common pressure drop of 2 inches of water (0.0722 psi), and a 32 inch square door with hinges 2 inches from the top and a handle 4 inches from the bottom, the force required to open the door would be 50 pounds, in addition to the weight of the door. Pressure drops of as much as 4 inches of water are common across doors near the intake or exhaust areas and the opening force required for these doors is proportionately larger. This excessive force requirement is particularly a problem for small employees or those who have been injured in mines. It is one objective of this invention to provide an improved pressure-balanced door that may be opened without excessive force.
The large pressure drop causes an additional problem after the door is opened. The air flow through the access opening can result in violent door closure. In typical operation, the door is propped or wedged open for use, and the last person through the opening reaches back to kick or pull out the wedge. The door closing, accelerated by air pressure (and also by gravity in an overhead hinged door), is very rapid and sufficiently violent to risk serious physical harm to operators. It is a further objective of this invention to provide a door between areas of different pressure that may be safely closed without violent force.
"Butterfly valve" doors which are hinged to pivot about the middle have been used in mining operations to eliminate the pressure load problem, but such a configuration requires a door and opening of twice the height of a standard door and, resultingly, nearly twice the potential leak path through the access opening. It is another objective of this invention to provide a door of minimum dimensions to reduce the potential for air leakage through the access opening.
Other, more complex pressure balancing doors, e.g. revolving doors or doors with sliding support or guide members, have been employed in mine operations in areas of high pressure differences, but more sophisticated designs are not compatible with dirty mining operations. It is an objective of this invention to provide a door of low cost and extremely simple configuration, that is easily installed, requires no precision manufacture, and avoids sliding support guidance members while providing a desirable degree of relief of opening and closing pressure load and, if desired, counterbalancing gravity loading.